Currently, if the number of your established BGP sessions is larger than FD_SETSIZE limit, then you will get 100% cpu usage on select() (returning EINVAL for nfds) after trying to include the first fd>=FD_SETSIZE.

Probably, the better way of handling this would be by explicitly rejecting active/passive sockets setup with proper error message so user could rebuild bird with appropriate limit without spending time on diagnosing 100% cpu usage issue.